1. Field of the Invention
The present invention relates to a microjet based control system for the control of a feedback loop created by an impinging jet of a STOVL aircraft.
2. Background of the Prior Art
When a Short Takeoff and Vertical Landing (STOVL) aircraft is in hover mode in close proximity to the ground, the lift force is produced by the downward pointing lift jets (impinging jets) which jets are located on either side of the aircraft and which produce a high speed hot flow that impinges on the ground. It is well known that under these conditions, several flow-induced effects can emerge which can substantially diminish the performance of the aircraft. Specifically, a significant lift loss can be induced due to flow entrainment by the lifting jets from the ambient environment near the airframe of the aircraft. Additional problems that can occur include severe ground erosion that can be occasioned from the downflow being issued by the impinging jets, as well as hot gas ingestion into the engine inlets. Furthermore, the impinging flow field usually generates significantly higher noise levels relative to that of a free jet operating under similar conditions. Increased overall sound pressure levels are associated with high speed impinging jets and these increased levels can pose an environmental pollution problem and can adversely affect the integrity of the structural elements in the vicinity of the nozzle exhaust due to acoustic loading. The noise and unsteady pressure fields created by the impinging flow are frequently dominated by high amplitude discrete tones which can match the resonant frequencies of the aircraft panels, increasing the criticality of sonic fatigue.
The presence of multiple impinging jets can potentially further aggravate the above noted effects due to the strong coupling between the jet flows and the emergence of an upwardly moving fountain flow that flows in opposing direction relative to the impinging jet flow, toward the aircraft.
Obviously these negative effects need to be controlled in order to minimize their adverse influence on aircraft performance. In order to arrive at an effective control scheme to minimize or eliminate these undesired characteristics, an understanding of the physical mechanisms governing these flows is required.
Early studies have found that the unsteady characteristics of the impinging jets are dominated by the presence of discrete impingement tones. These high amplitude tones are generated by highly coherent instability waves as a result of the emergence of a self-sustained feedback loop. Impingement on the wall of the large vertical structures will generate coherent pressure fluctuations, which result in acoustic waves that travel through ambient medium and upon reaching the jet nozzle (a region of high receptivity), excite the shear layer of the jet. This leads to the generation of a new set of enhanced instability waves, thus closing the feedback loop.
Other studies have found an intimate connection between the discrete impinging tones and the highly unsteady oscillatory behavior of the impinging jet column. Through the generation of large scale structures in the jet shear layer, the feedback phenomenon might also be responsible for lift loss on the surfaces in the vicinity of the jet nozzle. These structures induce higher entrainment velocities that lead to lower surface pressures in the jet vicinity and, consequently, a significant lift loss.
Further studies have found the emergence of discrete peeks in the spectra of the unsteady surface pressures which match the impinging tone frequencies in the near-field acoustic measurements. This suggests that these feedback loop driven flow instabilities are also responsible for the unsteady loads on the ground plane. Unsteady loads as high as 190 dB have been found, which, coupled with the high temperatures associated with lifting jets, can further aggravate the ground erosion problem.
It is apparent from the above, that in order to effectively eliminate these undesirable effects of the impinging jet flows, the feedback loop needs to be weakened or outright eliminated. Several potential candidates emerge for achieving optimal control of the flow. One method intercepts the upstream propagating acoustic waves so that they cannot complete the feedback loop. Another method involves manipulation of the shear layer (e.g., increasing the thickness of the layer) near the nozzle lip thereby reducing its receptivity to the acoustic disturbances. Yet another method involves exciting the shear layer using pulsating high energy sources to disrupt the coherent interaction between the flow instabilities and the acoustic field.
A few attempts have been made to suppress the feedback loop. One such attempt suppressed edge tones in low speed flows, which are governed by a similar feedback mechanism, by placing two plates normal to the centerline of the jet. A similar technique was used to attenuate the feedback loop by introducing a control plate near the nozzle exit. The control plate was able to intercept the upstream propagating acoustic waves in order to disrupt the feedback loop, hence weakening the formation of the large scale structure of the jet flow. This passive control approach recovered a maximum of approximately 16 percent of the lift loss and an approximate 6-7 dB reduction in the overall sound pressure levels. While these passive control techniques look promising, significant performance gains were confined to a limited range of operating conditions. This is due to the fact that a relatively small change in the nozzle-to-ground separation can lead to a significant change in the magnitude and frequency of the tones that are responsible for the undesired results.
Therefore, it is apparent that any efficient control technique aimed at suppressing the feedback loop must be active and capable of adapting to a shift in frequencies/wavelengths of the modes that lock onto the feedback loop.
One attempt used a high speed co-flow with the impinging jet flow to shield the main jet flow from the near field acoustic disturbances. This attempt saw a reduction of approximately 10-15 dB in the near field broadband noise level in addition to the suppression of impinging tones. While this attempt was effective, it required a high mass flow rate of the co-flow jet, a minimum of approximate 20-25 percent of the main jet mass flux. Another attempt used counterflow near the jet nozzle exit to successfully suppress screech tones of non-ideally expanded jets. This attempt achieved an approximately 3-4 dB noise reduction and enhanced the mixing of the primary jet.
However, each of the above active control schemes require substantial additional design modifications and/or high operating power rendering them impractical for aircraft implementation.